HomeMy WebLinkAboutCT 2017-0005; GRAND WEST; ADDEMDUM 01, TEMPORARY SHORING RECOMMENDATIONS; 2020-01-27
January 27, 2020 CTE Job No. 10-13643G Consultants Collaborative
Attn: Ms. Terry Matthew 160 Industrial Street San Marcos, California 92078 (760) 471-2365 Via Email: terry@cciconnect.com
Subject: Addendum 01, Temporary Shoring Recommendations Proposed Two Triplex Townhome Condominiums 972 & 988 Grand Avenue
Carlsbad, California
References: Geotechnical Investigation Proposed Two Triplex Townhome Condominiums 972 & 988 Grand Avenue
Carlsbad, California
CTE Job No. 10-13643G, dated May 31, 2017 Ms. Matthew:
As requested, Construction Testing and Engineering, Inc. (CTE) provides addendum
recommendations herein for temporary shoring for the subject project. CTE understands that excavation adjacent to the western property line (PL) is proposed for the construction of a storm water bio-filtration basin. Due to the proposed depth of the potential excavations (approximately six feet) and proximity to the PL, temporary shoring of the excavation is recommended, and as
CTE understands required by the City of Carlsbad. Additionally, these recommendations may be
used for the preliminary design of temporary shoring elsewhere on the project site, however CTE should be requested to review proposed shoring plans prior to construction. Recommendations are provided herein for shored excavations consisting of cantilevered soldier
piles with continuous timber lagging. CTE also recommends the client contact an experienced
shoring contractor to explore the feasibility of using braces/walers as a less expensive method of temporary shoring; however, CTE recognizes that braced shoring may pose potential constructability issues for the proposed bio-filtration basin.
1.0 TEMPORARY SHORING RECOMMENDATIONS
The shoring contractor should be experienced in the design and construction of similar shoring systems and demonstrate proven competence on projects of similar size and magnitude. The shoring designer and contractor shall anticipate encountering local layers of relatively
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cohesionless and un-cemented materials that may be subject to sloughing and caving. In addition, groundwater, debris, gravels, and/or cobble material may be locally encountered.
1.1 Lateral Earth Pressures
We anticipate that a temporary shoring system would likely be used where sufficient setbacks for sloping excavations are not available. Braced or unbraced shoring may be designed using the same active and at-rest soil pressures recommended in the referenced geotechnical report for permanent walls, but the values may each be reduced by 30 percent. In addition to the
recommended earth pressures, the upper 15 feet of shoring adjacent to streets or other traffic areas shall be designed to resist a uniform lateral pressure of 100 pounds per square foot (psf) that results from an assumed 300-psf surcharge behind the shoring due to typical street or other traffic. For traffic that remains more than 10 feet away from shoring, surcharge loading may be neglected.
Although the actual deflections of the shoring should be determined by the shoring engineer, shoring designed as stated is anticipated to deflect less than one inch at the top of the shored embankment. These deflections should be within tolerable limits for adjacent improvements such as buried pipes and conduits, or sidewalks and streets, provided these improvements are in
generally good structural condition. Friction tieback anchors and/or a greater active design pressure could be used to reduce the amount of deflection at the face of the shoring. CTE should review the final shoring calculations and drawings in order to identify potential conflicts with the recommendations contained herein. In addition, observation by this office will
be required during shoring installation activities. Monitoring of settlement and horizontal movement of the shoring system and adjacent improvements should occur on a weekly basis during construction in order to confirm that actual movements are within tolerable limits. The number and location of monitoring points shall be
indicated on the shoring plans; CTE will review such locations and the proposed monitoring schedule once prepared and provided by the shoring contractor.
1.2 Design of Soldier Beams
For conventional soldier beam and lagging shoring systems, soldier beams, spaced at least three diameters on center, may be designed using an allowable passive pressure of 300 psf per foot of
depth, up to a maximum of 3,000 psf, for the portion of the soldier beam embedded in competent underlying materials below the proposed excavation depths. Provisions should be made to assure firm contact between the beam and the surrounding soils. Concrete placed in soldier beams below the proposed excavation should have adequate strength to transfer the imposed pressures. A lean concrete mix may be used in the soldier pile above the base of the proposed
excavation. Soldier beam installations should be observed by CTE. Localized to widespread caving of onsite soils should be anticipated, especially if drilling/installing below the water table.
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1.3 Lagging
Continuous timber or pre-cast concrete lagging between soldier beams is recommended.
Lagging should be designed for the recommended earth pressures but be limited to a maximum pressure of 500 psf due to arching in the soils. Voids created behind lagging by sloughing of locally cohesionless soil layers shall be grouted or slurry filled, as necessary, as construction progresses. In addition, generally the upper two to four feet of lagging shall be grouted or slurry-filled to assist in diverting surface water from migrating behind the shoring walls. Localized to
widespread caving of onsite soils could be anticipated, especially if lagging is being installed below the previous/recent water table elevations.
1.4 Anchor Design
Cantilever shoring is anticipated to be adequate for the subject site/development. However, anchor design recommendations are provide should their use become necessary. For design
purposes, it may be estimated that drilled friction anchors will develop an average friction of 1,500 psf for the portion of the anchor extending beyond the active wedge and embedded in the effective zone. However, additional capacities may be developed based on the installation technique. Friction anchors should extend a minimum of 15 feet beyond the active wedge. However, greater depths may be required to develop the desired capacities. The active wedge is
defined by a 1.4:1 (horizontal: vertical) plane extended up and away from the bottom of the shored embankment. Localized to widespread caving of onsite soils could be anticipated.
1.5 Friction Anchor Installation
Friction anchors may generally be installed at angles of 15 through 40 degrees below horizontal. Anchors should be filled from the tip outward to the approximate plane where the active wedge
begins. The portion of anchor in the active wedge should not be filled with concrete. Localized to widespread caving of cohesionless soils may occur during tieback drilling and the contractor should have adequate means for mitigation.
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The opportunity to be of service on this project is appreciated. If you have any questions regarding this report, please do not hesitate to contact the undersigned.
Respectfully submitted, CONSTRUCTION TESTING & ENGINEERING, INC.
Dan T. Math, GE# 2665 Colm J. Kenny, RCE #84406 Vice President, Principal Senior Engineer
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No.84406
XP. 9/30/21